1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor, and particularly relates to a multilayer ceramic capacitor having a large capacitance, large capacitance even in a compact size, and high reliability.
2. Description of the Related Art
A gain capacitance of a multilayer ceramic capacitor is in a relationship of a formula (1) below.
C=xcex50xc2x7xcex5rxc3x97nxc3x97S/dxe2x80x83xe2x80x83(1) 
(C: capacitance (F), xcex50: vacuum permittivity, xcex5r: specific permittivity of dielectric material, n: number of layers, S: valid area, d: thickness of dielectric)
Accordingly, in order to increase capacitance, there are methods of making the dielectric layer thickness d thinner, increasing the specific permittivity xcex5r, increasing the valid area S and increasing the number n of dielectric layers.
However, since there is a limit in increasing the valid area to obtain a large capacitance in a compact size, a method of increasing the permittivity or making the layer thinner is generally used.
Owing to the problem of unevenness of tickness, there has been said that the limit of a thinner dielectric layer was 10 xcexcm or 5 xcexcm, but a product having a thinner layer than the limit has come to be produced due to development of production techniques.
Even if a chip capacitor of an extremely thin layer wherein the dielectric thickness is 3 xcexcm or less can be produced, however, there arises a disadvantage that it cannot stand a practical use because resistance of the dielectric is too low. Therefore, a method of making a dielectric particle diameter between internal electrodes less than the thickness of a dielectric layer and making the number of dielectric particles between the electrodes two or more has been taken in the related art. It is for placing a grain boundary phase and securing insulation resistance by making the number of dielectric particles between electrodes two or more. Note that the state that the number of dielectrics between electrodes is two or more means that a straight line drawn vertically from one internal electrode to an internal electrode next to it passes through two or more particles.
However, when the layer becomes still thinner and the dielectric thickness becomes 3 xcexcm or less, it is necessary to make the particle diameter 1.5 xcexcm or less in order to make the dielectric particles between the internal electrodes two or more, so there has been a disadvantage that the gain capacitance cannot be made larger.
Thus, capacitance per volume inevitably becomes small and that has been an obstacle of attaining a compact size and larger capacitance.
Note that, as described in the Japanese Unexamined Patent Publication No. 11-317322, a capacitor wherein a particle diameter of about 20% or more of dielectric particles constituting the dielectric layer of the multilayer ceramic capacitor have a substantially same thickness as that of the dielectric layer has been proposed. This publication discloses improvement of the CR product, which is a product of the capacitance and resistance of the capacitor, by applying such a configuration.
This publication, however, only discloses multilayer ceramic capacitors wherein an average particle diameter of the dielectric particles is same or less than the thickness of the dielectric layer.
The present invention has been made in consideration with the above disadvantages of the related arts and has an object to provide a highly reliable multilayer ceramic capacitor having further improved capacitance per a unit volume and a large capacitance even in a compact size.
To attain the above object, according to the present invention, there is provided a multilayer ceramic capacitor comprising internal electrode layers and dielectric layers, wherein an average particle diameter (R), in a direction parallel with the internal electrode layers, in dielectric particles constituting the dielectric layers is larger than a thickness of the dielectric layer (d). Note that the average particle diameter of the dielectric particles in the dielectric layers means an average particle diameter of the dielectric particles in the valid dielectric layer (which contribute to the capacitance) between a pair of internal electrode layers. The average particle diameter is an average not including dielectric particles in a dielectric layer of which parts do not contribute to the capacitance (for example, a dielectric layer arranged on an outer side of the dielectric layer in the stacking direction not sandwiched by the internal electrode layers).
Preferably, a ratio (R/d) of the average particle diameter (R) and the thickness (d) of the dielectric layer satisfies 1 less than R/d less than 3.
Preferably, a main component of the internal electrode layers is Ni or Cu. In this case, Fe is preferably segregated in the internal electrode layers.
In the present invention, a multilayer ceramic capacitor of particularly high reliability can be obtained even if the thickness of the dielectric layer is less than 3 xcexcm.
In the present invention, a dielectric layer comprises at least the dielectric particle and a grain boundary phase. An area ratio of the grain boundary phase on a section of the dielectric layer is preferably 2% or less.
The dielectric particle may have, for example, a core-shell structure.
In the present invention, preferably, the dielectric layer is comprised of dielectric particles, a grain boundary and grain boundary phase, a segregation phase (second phase) exists in the grain boundary phase, and the segregation phase contains at least two kinds of elements selected from Mn, Y, Si, Ca, V and W.
According to the present invention, there is provided a production method of a multilayer ceramic capacitor, comprising the steps of firing a green chip to be a capacitor element body comprising dielectric layers and internal electrode layers in a reducing atmosphere; and performing heat processing under an atmosphere of which oxygen partial pressure is higher than the reducing atmosphere; wherein an average particle diameter (R) in a direction parallel with the internal electrode layers in dielectric particles constituting the dielectric layer is made to be larger than a thickness (d) of the dielectric layer.
Preferably, a temperature of heat processing after firing under the reducing atmosphere is 1000xc2x0 C. or more. Also, an oxygen partial pressure at the time of heat processing after firing under the reducing atmosphere is preferably 10xe2x88x923 Pa to 1 Pa.
Note that in the present invention, the average particle diameter is defined as below. Namely, when the multilayer ceramic capacitor is cut at a section which is vertical with respect to the internal electrodes and passes both of the external electrodes, a straight line which is substantially parallel with the internal electrode layer is drawn at a center portion between the internal electrodes on this cut section, and when assuming that the number of particles crossing with this line is n (n is 10 or more), and a length of the line is L, L/n is the average particle diameter (R) in the direction horizontal to the internal electrodes.
In the present invention, due to the dielectric layer having the above configuration, a highly reliable multilayer ceramic capacitor having large capacitance per a unit volume and a large capacitance even in a compact size can be realized.
Also, in the present invention, even if the thickness of the dielectric layer is less than 3 xcexcm, it is possible to obtain capacitance of a high volume ratio of 100 F/m3 or more by obtaining the configuration wherein the largest particle diameter of particles is larger than a distance between the electrodes. This particle diameter can be realized by adjusting a dielectric composition, firing temperature and firing atmosphere. Also, sufficient insulation resistance can be obtained by performing heat processing under an optimal oxygen partial pressure after firing in a reducing atmosphere, so the reliability improves.